How to know the Unknown area in the octree

[Mohamed Ahmed]

Hello everyone,
I'm working on exploration of 3D environment I want find the frontiers(free node, next to unknown area). I call the srv /octomap_binary and use
My question is: How to determine the frontiers or the unknown Area?
I think iterating through leafs of the octree and calculate the distance between this free node and the edges of the desired area to explore

[Mohamed Ahmed]

this will help to get unknowns ourmap.getUnknownLeafCenters(l, min_point, max_point);
another solution is to check if the node in the map or not using ourmap.search();